1. Field of Invention
The field of the currently claimed embodiments of this invention relates to devices that incorporate ferroelectric materials.
2. Discussion of Related Art
Single crystal relaxor ferroelectrics, such as [Pb(Zn1/3Nb2/3)O3](1−x)-[PbTiO3]x (PZN-xPT, 0<x<0.1) and [Pb(Mg1/3Nb2/3)O3](1−x)-[PbTiO3]x (PMN-xPT, 0<x<0.35), exhibit unique dielectric and electromechanical properties. Previous studies focused on mapping the phase-composition diagram,[1-3] temperature dependence,[4-10] and electric field or stress induced phase transitions[11-18].
Park and Shrout [2] performed some systematic measurements on the piezoelectric properties of compositions of single crystals in both the PMN:PT and the PZN:PT systems. For composition in the ferroelectric rhombohedra phase field close to the morphotropic phase boundary (MPB), the piezoelectric coefficient d33 is larger than 2500 pm/V and electromechanical coupling k33 is larger than 0.94. Vieland and Li [14] reported anhysteric quasi-linear strain response and electric field-induced rhombohedral to orthorhombic phase transformation in <110> oriented 0.7Pb(Mg1/3Nb2/3)O3]-0.3[PbTiO3] crystals. Kutnjak et al. [10] showed that the giant electromechanical response in PMN-PT (and potentially other ferroelectric relaxors) is the manifestation of critical points that define a line in the phase diagram. Specifically, the paraelectric-ferroelectric phase transition in PMN-PT terminates in a line of critical points where the piezoelectric coefficient is maximum. Yiping et al. [15] reported domain configuration and ferroelectric properties of (011) cut relaxor-based PMN-PT single crystals. Within the morphotropic phase boundaries (MPB) region, three phase and domain configurations are present, denoted by “R-O”, “O” and “R”. The PMN-PT single crystals in all three configurations exhibit a giant electric field poling induced remanent strain. Particularly in the “R” composition region, a monodomain orthorhombic ferroelectric state can be achieved by applying an electric field.
U.S. Patent Pub. No. 2011/0017937 A1 describes <110> domain engineered relaxor-PT single crystals having a dielectric loss of about 0.2%, a high electromechanical quality factor greater than about 85%, and high mechanical quality factor greater than about 500.
The relaxor-PT single crystals may be formed using the Vertical Bridgeman method. The <110> single crystal has a strong anisotropic behavior due to a macroscopic mm2 symmetry as compared isotropic behavior of a 4 mm symmetry <001> poled crystal. The figure of merit (FOM) or d33Q33 for the <110> oriented crystals was much higher than the value obtained from <001> oriented crystals.
U.S. Patent Pub. No. 2003/0154911 A1 describes a method of growing single crystals of lead magnesium niobate-lead titanate (PMN-PT) near the MBP. The method includes providing a flat-bottomed iridium crucible containing PMN-PT starting material, and placing the crucible into a vertical Bridgeman furnace having at least two temperature zones. The starting materials include high purity (>99%) powders of Pb3O4, MgCO3, Nb2O5 and TiO2. Raw powders of the starting materials are weighed in desired molar ratios with excess Pb3O4 as a flux. The first temperature zone has a temperature higher than a melting temperature of the PMN-PT material, and the second temperature zone has a temperature less than the melting temperature of the PMN-PT material. The single crystals of PMN-PT prepared using this method exhibits a [110] crystal orientation. However, conventional devices that use ferroelectric materials require constant power to maintain the ferroelectric in a strained state. Therefore, there remains a need for improved devices that incorporate ferroelectric materials.
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